Hydrosilylation chemistry, involving the reaction between a silylhydride and an unsaturated organic group, is the basis for synthetic routes to produce commercial silicone products such as silicone surfactants, silicone fluids and silanes as well as many addition cured products including sealants, elastomers, RTVs, adhesives, and silicone-based coatings. Conventionally, hydrosilylation reactions have been catalyzed by precious metal catalysts, such as platinum or rhodium metal complexes.
Various precious metal complex catalysts are known in the art. For example, U.S. Pat. No. 3,775,452 discloses a platinum complex containing unsaturated siloxanes as ligands. This type of catalyst is known as Karstedt's catalyst. Other exemplary platinum-based hydrosilylation catalysts that have been described in the literature include Ashby's catalyst as disclosed in U.S. Pat. No. 3,159,601, Lamoreaux's catalyst as disclosed in U.S. Pat. No. 3,220,972, and Speier's catalyst as disclosed in Speier, J. L, Webster J. A. and Barnes G. H., J. Am. Chem. Soc. 79, 974 (1957).
Although these precious metal compounds and complexes are widely employed commercially as catalysts for hydrosilylation reactions including those employed in cure technology, they have several distinct disadvantages. One disadvantage of the current catalyst systems is the undesired color imparted to the final product. This yellow coloration or Pt precipitation in crude products often necessitates additional and costly purification steps. Another distinct disadvantage of the current systems is the progressive deactivation of the platinum catalysts during the course of the reaction which necessitates higher loadings of this costly metal. Yet another disadvantage encountered with Pt-catalyzed hydrosilylation of unsaturated and COH-terminated oligo- or polyethers is the undesired reaction of SiH with the alcohol OH, which produces SiOC linkages that waste SiH groups, leave unreacted C═C bonds, and can cause performance problems.
This need is particularly acute for release coating formulations for better performing catalysts at low Pt loadings, where perhaps the most stringent demand is placed on the catalyst for extremely fast cure at high line coating speeds and very short oven-dwell times (1-5 seconds), together with good bath life of the formulation. Yet, the formulation must essentially completely cure in seconds at elevated temperature to meet release performance requirements on a plethora of different paper and polymeric substrates. To accommodate these two opposing demands, two part formulations with high platinum loadings and high inhibitor loadings are typically employed in the industry. This current solution has several distinct disadvantages. High platinum catalyst loadings are required in addition curable systems to ensure rapid and complete cure at elevated temperature but this high loading of precious metal catalysts also imparts a significant catalyst cost to the formulation. High levels of inhibitors are employed to retard catalyst activity and to extend working life of the formulation at room temperature, but the inhibitors employed may not be rapidly decomplexed from the platinum center at elevated temperature and may slow the desired crosslinking reaction at elevated temperature.
Due to the high price of precious metals, catalysts derived from these platinum metals can constitute a significant proportion of the cost of silicone formulations. Over the last two decades, global demand for precious metals, including platinum, has sharply increased, driving prices several hundred folds higher, thereby precipitating the need for effective, yet lower catalyst loadings. There is a need in the silicone industry for platinum catalysts of improved stability. This improved stability would enable the lowering of Pt catalyst loadings and decreasing cycle time in reactors and improving yield for many hydrosilylations.
The use of pre-formed Pt-COD complexes (COD=1,5-cyclooctadiene) in hydrosilylation reactions has been previously reported, e.g., JP 54076530A, JP 54076529A, EP 472438, L. Lewis et al., Organometallics, 1991, 10, 3750-3759, and P. Pregosin et al., Organometallics, 1988, 7, 1373-1380. PtCODCl2, PtCODMe2, and PtCODPh2 are commercially available and their use as catalysts for hydrosilylation has been known for many years. Roy et al. have reported the preparation of a series of PtCOD(SiR3)2 compounds from PtCODCl2 (Roy, Aroop K.; Taylor, Richard B. J. Am Chem. Soc., 2012, 124, 9510-9524; and U.S. Pat. No. 6,605,734). Notably, the preparation of these CODPtSi2 complexes strictly requires the use of at least three equivalents of COD per equivalent of Pt, even when prepared in situ for hydrosilylation catalysis, as COD is lost to both hydrogenation and isomerization (1,4-COD and 1,3-COD) reactions. This critical stoichiometry of COD/Pt is delineated both in the '734 patent and the JACS publication. Further, the use of only one COD per Pt led to no identifiable COD-Pt species, as reported in the JACS publication.
The use of pre-formed Pt (II) cyclooctadiene catalysts in addition cure reactions has been reported. The use of PtCODPh2 has been reported for use in radiation cure systems in WO92/10529. Complexes with the general formula PtCOD(alkynyl)2 and Pt(COD)(ureylene) have been cited as catalysts in curable silicone rubber compositions, e.g., EP 0994159 and U.S. Pat. No. 7,511,110. These platinum II complexes, however, suffer from their poor solubility in organic solution and silicone formulations. Chlorinated solvents such as chloroform or dichloromethane are employed to dissolve the catalyst. In addition to health and environmental concerns posed by such chlorinated solvents, they are also highly volatile which poses formulation challenges.
The use of COD as an additive has been shown to reduce the amount of bis-silylated product in hydrosilylation of only alkynes with hydrochlorosilanes in U.S. Pat. No. 5,563,287. Other cyclodiene complexes of platinum are also known and commercially available, such as (norbornadiene)PtCl2 and (dicyclopentadiene)PtCl2, but again, these latter diene complexes are not known to provide any particular benefit over catalysts such as Speier's or Karstedt's.
There is a need in the silicone industry for platinum catalysts of improved stability as industry work-horse catalysts such as Speier's and Karstedt's are prone to partial deactivation via agglomeration, especially at elevated temperatures of use. Improved stability of the active catalyst would enable the lowering of Pt catalyst loadings. In addition to improved stability, catalysts that demonstrate rapid activation and high hydrosilylation activity at elevated temperature are especially sought. The present invention provides one solution to these needs.